The artificial incubation of birds' eggs is usually carried out in incubators in which the eggs are enclosed in a more or less sealed chamber in which the temperature and humidity are controlled in order to maintain the eggs at conditions which give incubation periods similar to the incubation periods associated with natural incubation. Such conditions usually provide optimum hatch rates in this kind of incubator. Such incubators have proved successful in hatching poultry and eggs of similar species, but less successful in the case of rare, wild or exotic birds.
However, the conditions in the aforementioned type of incubator differ in various ways from the conditions associated with natural incubation because, in natural incubation, the upper part of the egg is subjected to warming by contact with the brood patch of the parent bird, whilst the lower part of the egg is in a nest environment which is not at a closely controlled temperature. These natural conditions cause a temperature difference from the top to the bottom of the egg, unlike the uniform temperature found in most fan-circulated incubators. There may also be other important differences in natural incubation, for example the area of the egg covered at any time by the brood patch or in contact with the nest material, and the resulting restricted gas exchange through areas of the eggshell; as well as the periodic cooling caused when the parent bird leaves the nest to feed or defecate.
Attempts have been made to copy more accurately the natural incubation process by employing machines in which a controllable temperature gradient is imposed across the eggs, such as the incubator disclosed in British Patent No. 2230931 granted to F. H. Pearce. Incubators have also been proposed in which the eggs are heated by conduction from a warm surface mimicking the brood patch of the parent bird; in one such case (as disclosed in U.S. Pat. Nos. 5,542,375 and 5,638,771 granted to Voren) a rubber bladder or hot water bottle with water at a controlled temperature circulating through it, is placed across the top of the eggs which are contained in a shallow tray provided with wood shavings, to simulate a nest. The weight of the water-filled rubber bladder provides sufficient pressure to ensure good thermal contact between the bladder and the eggs. Turning of the eggs is carried out by hand, which necessitates removal of the rubber bladder from the eggs. Under natural conditions, eggs are turned during incubation, which is why it is desirable to simulate the turning during artificial incubation.
In another arrangement (as disclosed in U.S. Pat. No. 4,215,651 and United Kingdom Patent No. 2007957, both granted to F. H. Pearce) a flexible screen resting on the eggs is heated with circulating warmed air. The eggs are heated by contact as in the rubber bladder arrangement mentioned above, but the system employing the flexible screen is not particularly successful in view of the fact that the screen does not necessarily form a good thermal contact with the tops of the eggs.